1. Field of the Invention
The present invention relates to a method and apparatus for detecting an excitation position of an SRM by comparison of detected currents, and more particularly to a method and apparatus for detecting an excitation position of an SRM, which can determine the time of a phase excitation by simply comparing detected currents, without using a position detection sensor.
2. Description of the Prior Art
A switched reluctance motor (hereinafter referred to as an “SRM”) is a power drive device which can be easily manufactured, be inexpensively manufactured, and have relatively high reliability since it is proof against drive accidents. Hence, an SRM drive system has some characteristics comparable with an existing induction motor or a permanent magnet motor, in view of high torque, a high volume, a high output, a high-efficient variable speed drive, and an economic inverter power in applied fields such as industry machinery, airplanes, automobiles, consumer devices, and others.
Meanwhile, the SRM essentially needs the position of a rotor, owing to a motor driving mechanism. Specifically, the rotor and stator of the SRM have a double salient pole in order to maximize variable reluctance torque. The SRM receives information on the position of the stator, and properly implements the phase excitation to generate needed torque. A price of the encoder or resolver of the SRM is similar to a manufacturing cost of an original motor. Therefore, it is difficult to be applied to a simple rotor, due to such an expensive position detection sensor. Also, the high resolution encoder to be used for more precisely drive is not applied to the simple rotor.
The research and development of a conventional SRM have been implemented to improve torque, efficiency and reliability, and also reduce a cost. The development of the SRM has been concentrated on a proper magnetic circuit and a control strategy. Thus, a sensorless control having no encoder has been searched in order to reduce an expense, improve environmental stress-crack resistance, and obtain good stability. When a use range of the SRM is expanded into the consumer devices and other industry motor, the expensive position sensor is in the way. The following methods have been proposed to overcome the above drawback: a method of simplifying the structure of the position sensor; and a method of detecting a phase current, calculating an inductance from the detected current, and converting this into the position.
FIG. 1 is a graph depicting a process of detecting the position of the rotor through a conventional inductance estimation method. In general, the inductance estimation method applies a pulsed test voltage to the whole domains of the phase which is not excited, and calculates the inductance of the phase applied with the pulsed voltage according to a waveform of the detected current to be generated. An inductance of each phase is operated from the calculated inductance by use of a mechanical angle of the motor between the phases, thereby estimating a present position of the rotor. Other methods using the detected current are based on the inductance.
The above method may obtain a good effect if the number of phases of the SRM is more than 4, since there necessarily is a phase which is not excited. However, the conventional inductance estimation method has the following problems. First, it is difficult to apply the method, since all phases are overlapped in the 3-phase SRM. Second, the excitation voltage for generating the detected current has to be applied for a short period not to affect the drive torque. Third, since the inductance is calculated from the current detected by a current sensor and the inductances of the remaining phases are estimated based on the calculated inductance, a high performance operation system is required due to the complicated operation procedure. And, finally, the rotor has to be in rotation during the conventional sensorless control. Hence, each phase is excited in turn, and the rotor is rotated in any direction to detect and control the position. In this case, there is a safety problem due to a load condition, if reverse rotation has not to be implemented at start-up.